Her 6,700-strong network of students in New York has come under attack by the new mayor. But she's used to battling against politicians and their union backers.

For several months running, the Bill and Eva Show has been the talk of New York City politics. He is the new mayor, Bill de Blasio, an unapologetic old-school liberal Democrat, scourge of the rich and of public charter schools. She is Eva Moskowitz, fellow Democrat and educational-reform champion who runs the city's largest charter network.

How did Ms. Moskowitz, a hero to thousands of New Yorkers of modest means whose children have...